In a solar-thermal power plant, heliostats or collectors are used to focus the incident solar radiation onto a solar receiver which is commonly assembled from a large number of ceramic absorber bodies. The highly concentrated solar radiation incident on the absorber body is about 200 to 300 times stronger than the natural radiation of the sun; as a result, temperatures above 1000.degree. C. are generated in the absorber body. Due to such high temperatures, metals are not useful as materials for the absorber. Instead, use is made of highly temperature-resistent ceramics, e.g. silicon carbide or silicon silicon carbide.
Basically, two constructional designs are known for absorbers: that of a volumetric absorber and that of a direct-absorption absorber. Volumetric absorbers comprise absorber arrays made from metallic or ceramic honeycomb structures, reticulated structures or foams. The absorber arrays will absorb the solar radiation while having a fluid, e.g. ambient air in case of an open volumetric absorber, flowing therethrough. The fluid is convectively heated and supplied to a subsequent process such as a steam-turbine process. In absorbers for direct absorption, dark particles conveyed in a gas or a liquid are exposed to the solar radiation so that the particles will absorb the solar radiation and thus heat the gas or the liquid.
The source of energy of solar-thermal power plants is the radiation of the sun which, in contrast to the energy sources of conventional steam generators, is not adjustable and is subjected to continuous change depending on the position of the sun as determined by the time of day and the seasons of the year. The latter factors cause changes both in the intensity of the incident radiation and in the distribution of the incident radiation on the solar receiver. To utilize the energy in the heat-carrier medium in an optimum manner, the outlet temperature of the heat-carrier medium across the cross section of the receiver should be as constant as possible.
For this purpose, the mass flow distribution of the heat-carrier medium across the receiver surface has to be adapted corresponding to the distribution of the intensity of the incident radiation.
Usually, an active control of the mass flow distribution of the heat-carrier medium is precluded by the high temperatures possibly in excess of 1000.degree. C. and by the costs incurred. As a result, a homogeneous outlet temperature can normally be obtained only for one design point. The design point of the mass flow distribution is normally selected to obtain an energetic maximum, averaged over the year, for the respective design. This means that the design is based on an average distribution of the incident radiation. Deviations of the actual distribution of the incident radiation from the above average distribution tend to cause local overheating of parts of the receiver. In practice, such incidents lead to a decrease of the irradiation onto the solar receiver so that the plant will not be able to utilize the maximum of the potentially available concentrated solar radiation.
Known from DE 42 23 779 C1 is a solar plant comprising an absorber body adapted for suctional intake of ambient air which after intake will be convectively heated by an absorber body. The absorber body has tubular channels extending therethrough for infeed of preheated air from the rear of the absorber so that the preheated air is guided through the absorber to a region externally before the absorber body. In this region, the preheated air mixes with the ambient air and is sucked into the absorber to be further heated therein. It has been observed that the preheated air discharged from a certain number of tubes has a high discharge velocity so that a considerable part of the preheated air will flow beyond the intake region of the absorber and therefore not be available anymore for the process. As a countermeasure, baffle bodies can be arranged in front of the tube outlets for deflecting the discharged preheated air towards the absorber. Since this region of the solar plant is exposed to very high temperatures, the design and the attachment of these deflecting bodies are very complex.
It is an object of the invention to increase the efficiency of a solar receiver, particularly in that the receiver can be used over a wide range of temperatures.